Thermographs of the type for monitoring perishables and the like have been characterized by clock movement drives and bi-metal thermometers, both of which are subject to unreliability, the former susceptible to abuse and the latter prone to inaccurate calibration. Clock movements are delicate at best, they involve a considerable amount of mechanism and are inherently weighty with accuracy always in doubt. Therefore, it is a general object of this invention, to provide a most durable thermograph not subject to normal abuse or inaccuracy, and accordingly this reliability is accomplished by use of solid state electronics and primary motor drives.
Heretofore, the functions of temperature readout and time advancement have been independent, although associated in the one instrument. With the present invention, it is an object to cooperatively associate the time and temperature functions whereby one is dependent upon the other. Battery power is feasible at refrigeration temperature (40.degree. F.), and also at freezer temperature (-10.degree. F.), and it is an electronic circuitry and stepper motor drive for operation at both refrigeration and freezer temperatures which is an object of this invention. With this invention, a time base is established electronically and employed for both chart advancement and stylus positioning. In other words, solid state electronics is employed in a logic applied to both time and temperature, and involving generally means for establishing a time base employed to actuate a prime mover to incrementally advance a chart and also to trigger means to actuate a prime mover that positions a stylus according to prevailing temperature.
Physically, size and weight is of prime concern due to the ever increasing costs of transport, and to this end it is an object to provide a small light weight instrument, and in this respect the major functions of this instrument are incorporated in a light weight embodiment controlled by a printed circuit board utilizing solid state elements and integrated circuits. And being electronically oriented, the chart prime mover and stylus positioning means are electrically operated from a battery power supply and preferably by means of stepper motors. In practice, a nine volt alkaline battery is employed and the stepper motors are 100 ohm solenoids that operate ratchet pawls for angular displacement of gear transmission means to a chart take-up spool and recording stylus, respectively.
Longevity of the instrument use is of prime concern, being implemented for use periods extended from several days to a month or more. That is, the expected use is continuous and is repeated several or more consecutive times during the life of the instrument. This light weight instrument has a small power supply in the form of a battery from which energy is taken for instrument operation, and consequently conservation of this power is an important factor and a primary object of this invention. To this end the instrument is operated at widely separated intervals by momentary pulses of short duration, for example at two minute intervals with 30 millisecond pulses, and to the end that the operational time is 1/4000 of the total "ON" time. Further to this end, semiconductors of a type requiring minimal power for instrument operation are employed, namely Complementary Metal Oxide Semiconductors referred to generally as CMOS system integrated circuits or IC's having minimum voltage characteristics, and with the result that there is minimized battery drain for total system operation.
It is an object of this invention to improve the accuracy of thermograph recordings, with the realization that bi-metal thermometers have their limitations with respect to calibration. At best, a commercially produced bi-metallic element has a tolerance of a percent or two within the range of its operation, and this factor alone accounts for unpredictable discrepancies in excess of several degrees in temperature readout. For example, a bi-metal thermometer calibrated at one temperature is likely to be out of calibration at another temperature, and this discrepancy will increase with the differential between temperature readings. On the contrary, electronic response to temperature change can be far more accurate and linear with respect to temperature differential, and calibration made accurate at all temperature readouts within the range of the instrument. Although the instrument under consideration is intended for accuracy within one degree F., it is to be understood that far greater resolution can be attained with the concept herein disclosed.
It is another object to provide a positive stylus drive for recording temperatures, which obviates readout discrepancies incurred with the usual flexible bi-metal means of the prior art. Heretofore, chart movement has adversely affected stylus position, due to frictional engagement of the stylus arm angularly disposed to chart movement. With the present invention, stylus position is a purely electro-mechanical function in no way affected by chart movement and frictional engagement of the stylus therewith.
It is still another object of this invention to provide a thermograph that responds to the prevailing temperature with servo positioning of the thermometer means, and conceptually a electronic combination comprised generally of a sensor that produces a voltage responsive to temperature and a transmitter that produces a voltage responsive to position of the thermometer means. In accordance with this invention, these voltages are compared and the nature of any error detected and discriminantly switched to drive means for positioning of the thermometer means to correspond to the prevailing temperature.
Thermographs of the type under consideration are prepared for logistics that involve the transport of perishables from grower-shipper to receiver via a public carrier. Generally, the goods are sent by truck or rail and also by boat or air to a consignee who must hurridely inspect the shipment before acceptance; and it is the temperature condition during transport which is a determining factor when there is a question as to continuity and degree of refrigeration during the trip. Battery operated pre-loaded instruments have been provided for this purpose, that is instruments loaded with chart paper and prepared to operate when desired. It is an object therefore, to provide a pre-loaded instrument that automatically starts to operate when closed and sealed, that continues to reliably operate until opened and from which the chart recording is readily withdrawn for viewing, and that automatically ceases to operate when the seal is broken for removal and access for said chart removal, and further an instrument which is then adapted to economical mail return to a refurbishing service to be prepared for reuse.
An object of this invention is to provide a chassis based instrument adapted to be prepared and/or serviced and installed in a shipping and mail-back housing or container that is expendable. In practice, an injection molded plastic box and cover is provided, into which the instrument chassis is protectively installed. It is also an object to seal this box and cover combination with a metal strip or conventional car seal that assures instrument operation when installed, and to stop the instrument when removed. To this end, a manually depressible actuator is accessible through the cover and which requires depression for insertion of the said car seal. It is also another object to provide a recording instrument that carries with it a mounting bracket from which it can be readily removed; a hinged bracket and adhesive pad that is expendable and from which the instrument is removed as a unit with facility.